The present invention relates to acoustic sensing and measurement systems, and particularly to systems which detect the presence or a specific property of material bounded by a wall. Examples include systems for sensing liquid level and interfaces in a tank, fluid flow in a conduit, or the presence or thickness of ice built up on a hull, wing or fuselage.
Extensive technologies have been developed to measure flow rate, mass flow rate, or related parameters such as density or temperature, for fluids contained within conduits, by propagating ultrasonic waves along a path through the fluid, and measuring transit time, Doppler shift or other characteristic of the interaction of the signal with the fluid.
In many of these applications, while certain corrections must be made in the initial set up or subsequent signal processing to account for the effects of transducer mounting and housing geometry, the basic processing involves the measurement and comparison of ultrasonic signals propagating through a fluid. See, for example, U.S. Pat. No. 4,787,252 of Saul A. Jacobson et al. In other systems, wave energy may be propagated through a specially shaped wave guide, and the properties of the interrogating wave are affected by an interaction of the wave guide with a fluid that fills or surrounds the wave guide. U.S. Pat. No. 4,893,496 of Haim H. Bau et al. shows a system of this latter type, in which a polygonal or other specially shaped rod or cylinder is excited by a torsional wave, and coupling of energy from the rod or cylinder into an adjacent fluid provides a direct indication of the magnitude of the fluid's density or viscosity. In systems of this type, the wave is guided by the solid body, and its propagation is affected by energy coupling with the adjacent fluid.
It has also been suggested to use specialized systems of various kinds with flexural wave excitation to measure a static condition such as fluid height or ice covering.
In particular, work has been done in France by Dieulesaint on a fluid sensor in which a tube extends down into the fluid and is excited at one end with a flexural wave that is reflected at the air/fluid interface and then detected. The relatively slow propagation speed of a flexural wave in a long tube (or thin strip according to a method reported by Ageeva in 1960) allows resolution of the fluid level height.
U.S. Pat. No. 4,461,178 of Jacques Chamuel shows a method of detecting ice accumulations and the degree of attachment of ice formed on an aircraft wing. That system uses acoustic signals that propagate in both compressional and flexural modes through a sheet forming a surface of the wing. The patent reports that amplitude of the flexural wave varies with surface deposits, while the compressional mode propagates through the sheet unattenuated, and can therefore be used to normalize the amplitude of the received flexural wave signal.
In general it may be said that the systems of this latter type, involving the interaction of a flexed body with a surrounding material, appear to be rather specialized, and while distinct qualitative effects have been observed on the guided flexural wave signal, such systems do not have the well-developed theoretical models enjoyed by more conventional systems that employ unguided waves propagated through the fluid itself. Their teachings accordingly are limited to rather specific constructions.
It is therefore desirable to develop a more general wave detection or measurement system for elastic waves in sheet structures such as conduit, tank or vessel walls.
It is also desirable to develop an elastic wave system that is not restricted to use with customized or particular sensors, conduits or containment vessels.